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§ Accelerated Tests 

for the Settling of Pigments 
in Paints 

New JerseyN 



Issued by 

The New Jersey Zinc Company 

(Established 1848) 

160 Front Street, New York 


• I 

76 £ -M 

Accelerated Tests for 

the Settling of Pigments 

in Paints 


Sidney Werthan 

R. H. Wien 

Research Division. The New Jersey Zinc Company 

Heprmted April, 1930 

The New Jersey Zinc Company 

(Established 1848) 

160 Front Street, New York 

Products Distributed by 

The New Jersey Zinc Sales Company 

NewYork. Chicago Pittsburgh Cleveland San Francisco 


Research Laboratory, The New Jersey Zinc Company, Palmerton, Pa. 

The New Jersey Zinc Company 
is f?lad to make this contribution i 
as a part of its work in tin 1 interest - 
of the paint industi j . 


r^iIIKI.I i of products has always been one of the painl manu- 

^^ facturer's problems Although there is ;i considerable quantity <■!' 
informative data available on this subject, im satisfactory method 
foi determining the settling tendencies <»(' paints has been offered. 

Mo8l <»f the published data are I h« * results of observations of natural 

settling over extended periods <>(' time. I 'his procedure, however, i^ a 

discouraging undertaking because <»(' 1 1 m • slow progress and delayed re- 

In othei cases, attempts have been made to accelerate the settling 
h\ various methods, such as decreasing thi pigment-vehicle ratio l<\ 
the addition of vehicle decreasing the viscosity <>f tin- vehicle with 
extra volatile thinner or increasing the effective speci t> of the 

i i ntitii mixture I » n eliminating a light constituent. Vlso, settling tests 
have been suggested which are based on Mm- omission from a formula 
of a flocculating material <>r one thai possessed suspending proper! 
Bui ;m> procedure which depends on a change in the formulation of 
the producl under examination is not entirely satisfactory •»[ suitable 
Cor general application. 

According to the premises of the following papei settling is primarily 
due t<> differences in gravitational forces on the various constituents. 
The rale of saponification and the hardening or caking <<l the settled 

ment, which is an important phase of settling difficulties, is affei ted 
primarily l>> heal . 

Willi these facts in mind, an accelerated settling tesl has been de- 
veloped which involves a combination of centrifuging . m « i heating. The 

imples are kept in small glass i irs so thai the amount of settled 
menl and ihf supernatant liquid can be measured and an) gel or >kin 
ed. These jars are placed in a temperature cabinet where 
the> are kepi .ii 65 i" 70 i all of the time except i In- twent) minutes 
in the hi I evening when the) are centrifuged at L000 r.p.m. 

This acceleration routine was developed on flat wall paints because, 
uiili the possible exception of certain specialties, these paints as a rule 
offer more settling difficulties than «>iher types. This method should !>•■ 
applicable, however, with suitable modifications h»an\ paint, enamel, or 

[uer, In Hat wall paints, the effects of loi Is of natural kg 

have been obtained ii - 1»\ application of this acceler- 

iting routine 


The results of the accelerated tests have been compared will] those 
obtained through natural aging. 

To further check the effectiveness of these accelerated tests, com- 
mercial paints of known settling tendencies were subjected to the above 
routine for seven days. This test was found to be equivalent to more 
than two years normal aging and the accelerated results agreed with 
those obtained under normal conditions. 

After the samples have been subjected to this routine the condition 
of the settled mass is determined by a penetrometer, the unit of measure 
being the weight required to produce complete penetration bv standard 
plungers within thirty seconds. 

A method for determining the work required to reincorporate the 
settled pigment is also included. The stirring apparatus described has 
successfully broken up and reincorporated the hardest types of caked 
pigment. The time of stirring required to produce a smooth paint mix- 
ture is an indication of the hardness of the settled mass. This is con- 
sidered a valuable addition to the settling test inasmuch as it is of com- 
mercial importance that a settled paint reincorporate readily. 

It is possible to study the causes of the settling through microscopic 
examination of the caked pigment. This phase of the investigation is 
still under way. 

the first presentation of this paper the method described has been 
used extensively for determining the settling tendencies of various tvpes 
of flat waU paints Uthougb the settling produced by the test is generally 
typical <,f that which occurs when the paint is stored ,,,,.1,, normal eondi- 
SdLZ an n ? ccasiona ! I't" the ' "'""'■' l results maj be decidedlj 

1 * h. , ■ , Wed and natural settling 

usually, ,c,w> with flat wall paints possessing false bodies, induced bv 
Z I, W^T"?' This type of paint maintain, it. false bods if not 
agitated but readilj breaks down when stirred. II,,,,, disturbed, the pig- 
ment ■„ such a pamt ma> remain in suspension al,„..,i hul.-linit.-K.hul if 
occasonally moved, jarred, or shaken, objectionable settling often occurs. 

natmlTL u- £" paper produces resulte " Sparable to 

occur '" '""""' '"-""•"' oi handling of the paint 


Accelerated Tests for 
the Settlingof Pigments in Paints 

By Sidney Wkrthan and R. H. Wii \ 

Resear* h Division, The New Jersey Zinc Company 1 

ANY test by which the settling tendencies of a paint can be deter- 
/A mined before it is parked and distributed should be of value to 
the manufacturer. In this paper the various factors effect in*: the 
settling rate or tendency are briefly considered, accelerated tests based 
on these factors an 1 described and methods for examining lie degn e and 
nature of the settled rake are given. Although the settling of pigment 
in barrels, drums, dipping tanks and the containers used with spraying 
and other paint application equipment often causes difficulty, this paper 
is limited to the discussion of settling in small size packages or so-called 
"Shelf Goods." 

In the development of an accelerated settling test, one must first 
consider the factors involved and the variations in these that promote 
settling tendencies. These factors are the properties of the pigment and 
of the vehicle, pigment-vehicle ratio, type of container and storage 

Some of I he properties of the pigment that influence settling tenden- 
cies are the size of the particles or the relative particle sizes of the vari- 
ous components of the pigment mass, the dentil > or relali\e densities, 
and the activity which includes such properties as basicity, wettability, 
soap forming, flocculating, dispersing and gel forming tendencies. 
Those properties of the vehicle, variations in which influence settling 
tendencies, are specific gravity, viscosity. \ olatile-non-volatile ratio, 
and activity, which includes acidity, stability, wetting, dispersing and 
flocculating properties. 

Changing the pigment-vehicle ratio, that is, increasing the vehicle 
content is one of the simplest means of promoting settling. Although 
the type of container does not influence settling, the use of tall cylin- 
drical jars magnifies the relatiYe differences between samples. Both rate 
of settling and subsequent caking, increase with heat. \s settling is 
primarily due to the differences in the gra\ Maternal forces on the various 
constituents of the paint, any increase in this force will hasten settling. 

'Revision of a paper presented before the Division of Paint and Varnish Chemistry at the 7.">ih Meeting 
of the American Chemi* d Society, St. Louis, Mo.. April 16 to 19. 192B. 


Series of Flat Wall Paints containing addition;! I Mineral Spirits 

Mixture of Pigment and Vehicle Prepared Painl 

Comparison Between Rapid Test in which Pigment and V .hide are just Mixed and Test 

i ng Prepared Paints 


Based on these different factors, various tests have been developed. 
The rate of settling in a paint can often be accelerated by eliminating 
one of the pigment constituents, as. for example, a coarse, fibrous pig- 
ment or one of low specific gravity such as fibrous talc) or one that 

has suspending properties due to soap forming tendencies isuch as zinc 
oxide) or one that reacts wit li the vehicle to form a gel (such as aluminum 
stearate). Tests of this kind are usually quite limited in their applica- 
tion and should be adopted only after careful preliminary experimenta- 
tion has proven that the change in the pigment formula has only re- 
sulted in accelerating the normal settling of the balance of the pigment 

Perhaps the simplest means of testing a specific formula is to alter 
the vehicle or the pigment-vehicle ratio by adding extra thinner-. The 
addition of thinner has a threefold effect; it increases the vehicle con- 
tent, decreases the specific gravity of the vehicle and decreases it-- vis- 
cosity. Decreasing the pigment-vehicle ratio of a series of flat wall 
paints from 71-20 to 60- to by the addition of mineral spirit s made it 
possible to obtain settling indications within ten days thai were fairly 
representative of the settling in the regular products alter siv months' 

\ \< i \ simple application of this type of test is to just mix, by shaking 
(without an\ milling) the pigment with the reduced vehicle. Some- 
times the results so obtained are misleading, so it is preferable to add 
extra thinner to the regular paints. Still, with certain formulae, the 
same relative results weir obtained whether extra thinner v\as added 
to the paint or the pigment merely mixed with the reduced vehicle. The 
results were also comparable with the settling in the regular paints after 
prolonged storage. 

Rate of settling ma> he increased b> eliminating a flocculating con- 
stituent, such as an emulsifying agent from the vehicle. Uso increasing 
the dispersing properties of the vehicle by the addition of a small per- 
centage of pol\meti/ed oil or a thinner possessing high solvent powers 
has a similar effect. Vs in previous methods discussed, any test based 
on increasing the dispersing properties of the vehicle necessitates pre- 
liminary investigation to establish the value of the results obtained. 

\ test suitable for general application should not necessitate altering 
the product under examination. Objectionable settling is the combined 
result of migration of the pigment toward the bottom of the container 
and reactions within the paint . Tin- cenlrifuge offers the simplest means 
of increasing lire rate of migration of the pigment and heat is the most 
logical agent for accelerating the activities that produce caking. The 
rate of migration can be controlled by regulating the speed of the cen- 
trifuge, while, b\ a careful selection of temperature, the maximum ac- 


INI \l\* JKKSK^ /IN< «OMP\N\ 







celeration of normal aging can be produced. The mosl promising accel- 
erated h'Ni consists of a combination of centrifugal force and heat and 
the perfect test depends on obtaining the corred balance. The ideal 
condition would be hi appl\ I he corred centrifugal speed, under the 
proper temperature, which would uecessitate a controlled temperature 
room or cabinel for the centrifuge. Satisi a l> ( ' obtained 

by intermittently centrifuging the warm sample. If is necessary to de- 
termine a suitable cycle, care being taken thai a temperature is uo1 
used that is sufficiently high to cause reactions in the painl which nor- 
mally would not occur. The cycle chosen will generally depend on the 
equipmenl available and the degree of acceleration desired. 

This laboratory uses a twenty-four hour cycle, which i- quite severe 
is "iil\ four to five days are required to reproduce conditions typical 
of two years' normal storage. Tests are made in small glass jars vvith 
metal covers. These jars are approximately six centimeters in diameter 
and eleven centimeters tall with a capacity of L80 c.c. This jai was 
arbitrarily chosen because of the size of the cups on the centrifuge. In 
the presenl cycle the jar of painl is kept in a warm oven 65 70 < 
overnighl (approximately eighteen hours). The following morning the 
warm sample is centrifuged lor twenty minutes al 1000 r.p.m., returned 
to the oven lor approximately five hours, centrifuged again foi twenty 
minutes and. unless it is to he examined, the cycle i> repeated. \ sample, 
before being examined, is broughl to room temperature. Uthough mosl 
of the work has been with Mat wall paints, the results indicate thai this 
(est is applicable to other paints, enamels or lacquers. Sufficient tests 
i I' other paints and enamels have not been made to determine whether 
the cycle and factor used in testing flal wall paints can be applied pro- 
miscuously in the examination of miscellaneous products. 

I he usual means of examining and tire terms used for describing the 
degree of settling in a painl are often indefinite and inaccurate. The use 
of glass jars makes it possible to accurately measure the amounl of 
settled pigmenl or supernatanl clear liquid and eas> i«> detect an> ob- 
jectionable condition, such as a layer of gel or skin. I he amounl is 
usually no1 of as greal importance as the condition of the settled ma- 
terial. The penetration tests used for asphalt, greases and paste paints, 
are not suitable for examining the settled material in a paint, which 
often varies from a soft [taste in the upper portion to a hard almost 
,|i\ layei a1 the bottom. \ special penetrometer is used which consists 
of a vertically supported rod with a pan at the top for weights and a 
small plungei attached at the bottom. Three different plungers, which 
\.n\ m shape .nid size bu1 ht, are used. The tip of the 

plunger i- broughl in contacl with the top surface "I' the -"tiled material. 
released and i he distance t ra\ eled in thirty seconds recorded. II' complete 
penetration d<>e> not result in thirl \ seconds the tesl i> repeated with 


THK M W II HM 'i /l\< « o\|P\\\ 



•) • 


I III M W |Hts| \ /|N< < OMI'W* 

( lomparative Results between Natural Settling and Accelerated Settling 
Photographed 3 20 5 

l ill- l.rtu.'.-ri V.lunil Srlllin<r mui \. . . -|. , ,, i, ,| ^n Im- 
Photographed I 20 28 



the weight increased. Experience demonstrated that the movement of 
the plunger after thirls s* i onds is inappreciable and if complete pene- 
tration is not obtained in thirty seconds, additional weight is necessary, 
so as ,ui expediency thirty seconds pted as standard. Tests 

continued choosing a new spot for each lest until complete penetration 
i- obtained. B\ determining I he load necessary to penetrate l fie settled 
cake with al least two of the plungers, a measure i^ obtained of the 
relal ive degree of packing. \ penetrometer has been devised to automat- 
ically record the rial are of the settled pigment layer throughout its depth . 

In addition to ilie amounl of supernatant liquid and the hardness 
of ihe settled cake, the work required to reincorporate the settled pig- 
ment and the possibility of obtaining a smooth producl are of impor- 
tam e. In ihis laboratory this information is obtained with a special ^lir- 
ring apparatus. Three differenl mixing actions are produced with ii 
a rotation of ihe stirrer on iis own a\i>. a rotation of the stirrer around 
id' longh ud i nal avis of I he glass jar and a vertical up and down motion 
of Ihe stirrer. \ \<t> rapid and thorough mixing i-> obtained in a few 
minutes and even the hardest type of raked pigmenl can be reincor- 
porated. The time required to produce a smooth producl < -an be used 
i i measure of i In- w ork necessary . 

Hiii- of the greatest objections to settling in a painl i^ thai often 
when mixed b> ihe consumer a speck} producl i> obtained. I suall> 
tin- i> due to improper or insufficient mixing. When duplicate samples 
are available tlii^ machine affords an excelienl means of determining 
whethei i peck) product would be the result <>f improper mixing oi 
actually due to granulation of the paint. One of the samples i> tested 
l»\ first pouring off the supernatant liquid and adding il gradually dur- 
ing mixing, while the other is mixed direct. \ speck) producl in both 
'- definite indication of granulation in the paint. 

Another property of interest is the condition of the settled pigment, 
thai is, whether ii is dispersed, flocculated, aggregated <»r gelled. This 
information <;m be determined l>\ microscopical examination. Indica- 
tions are lhai mi< roscopical investigation of the whole subject of settling 
offers rich possibilities. Simple examination of ihe pigmenl in differenl 
portions <»t" a settled painl often furnishes valuable leads a^ to the cause 
of settling and possible means of improving ihe product. Although a 
larg< number of settled paints have been microscopically examined in 
this laboratory, ihi> subject has not been sufficiently investigated io 
w ai i ant definite conclusions. 

In I'ahlr. II. Ill and l\ are results of accelerated settling of three 
typical Hal wall paints, using a twenty-four hour cycle of heating and 
centrifuging. Paints "A" and "B" have fair non-settling properties and 



normally would not settle to an objectionable cake within a year. Paint 
"C" is not as satisfactory in this respect, settling more rapidly and to 
a harder cake. 

Table J covers tests on samples of the same paints which had settled 
during normal storage. They do not represent a sufficient number of 
different periods of aging to perhaps justify drawing definite relations 
between shelf storage and the accelerated cycle. Still, a comparison of 
the results of the Penetration Tests (ignoring the results on the sample 
of paint B which was tested after eight months' storage and apparent I \ 
falls somewhat out of line) one finds that each twenty-four hours of the 
cycle represents approximately five months" shelf storage. 

During the past two years this tesl has been used quite extensively 
in this laboratory in studying both the effect of the properties of pig- 
ments and variations in paint formulations on the settling tendencies 
of paints. The information obtained has been of appreciable value. 
Duplicate tests and actual shelf storage in a number of cases have 
shown the accelerated results to be both indicative and reliable \ 
large number of commercial Flat Wall Paints have been examined. Table 
\ contains the results on six representative first grade Hal wall paints. 

From the results the paints would be graded for non-settling proper! ies 
in the follow bag order: \ I he besl . C, F, K. and B and D 1 1 he poorest ■ 
Paints \ and C ha\e excellent non-settling properties and neither 
should cake bad!\ when stored from one to two years. Painl I has good 
non-settling and should cause no trouble unless held ovei a year. Paints 
E, B and I) all have poor' non-settling properties and will settle to an 
objectionable amount within a year. E packs slight l\ harder than B 
and D during the first few months but then slows up - i that at the end 
of a year or two it should be in better condition than the other two. 
There is \er> little choice between B and I). 

- feral outside paints imels, tlmi w< vn, from previous 

observations, not to develop settling even after several years oi -tor- 
age, were tested. No settling w;t- obtained in an> case, aftei seven d i 
w hen the tests were discontinued. 

It is not claimed that eithei .1 perfected test 01 an ideal 1 y< le has been 
de\ eloped, but it is believed that the results obtained definitely demon 
si rate that a test appl> ing heal and centrifugal fora Is an excellent tool 
witti which the settling tern paint can be determined. 



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Assembly of Equipment used for Determination of Accelerated Settling ol Paints 


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